In the field of optics, a combiner is an optical apparatus that combines two images together, from either the same side of the combiner (reflective/reflective, or transmissive/transmissive) or from the two different sides of the combiner (reflective/transmissive). Often times, optical combiners are used in heads up displays (“HUDs”), sometimes referred to as head mountable displays (“HMDs”) or near-to-eye displays, which allow a user to view a display image superimposed over an external view. The HUD enables the user to view the display image without having to look away from her usual viewpoint. The term HUD originated from its use in avionics, which enabled a pilot to view information while looking forward with his head up, as opposed to looking down at an instrument panel. Conventional HUD combiner implementations include tilted dichroic plates, holographic combiners, transparent substrates used at an angle (e.g., beam splitters), and zero power shells. Two versions of combiners exist. The first version combines two fields without adding any optical power to either field (typically a tilted dichroic plate or zero power shell). The second version has optical power, in addition to the combining functionality. The optical power is used to form a virtual magnified image of a microdisplay located at a specific distance from the combiner. Field of view is set by the application requirements. The field of view and the desired microdisplay diagonal determine the focal length (inversely related to the optical power) of the optical combiner.
The above mentioned optical combiners each have their own respective drawbacks, which have limited their use to niche markets. In order to broaden the adoption of an HMD outside of a particular niche market, such an HMD would ideally be implemented with a low cost optical combiner that is compact, lightweight, durable, and provide a good quality image without undue optical aberration.